Studies of the host gene Rmcfr, which specifically depresses in vitro and in vivo replication of MCF viruses, indicate that while this allele can suppress spontaneous and AKR MCR-induced thymic lymphoma in crosses of AKR and Rmcfr mouse strains, and results in lowered frequency and extended latent periods for Friend virus-induced erythroleukemia, it is much less effective in controlling T-cell lymphomas caused by Moloney or Gross Passage A virus. Rmcf alleles have been shown not to affect pristane-induced plasmacytomas in BALB/c mice. As part of a long-term program to analyze the effects of murine retrovirus associated genes on hematopoietic system tumors, both spontaneous and those induced by chemicals or virus inoculation, a number of genes of interest are being bred onto inbred mouse backgrounds. The majority of congenic lines are being established on the NFS Swiss mouse background, providing a standard background which is negative for its own endogenous ecotropic virus. Lines include the V-loci congenics (carrying ecotropic virus loci from AKR, C58, or C3H/Fg; and mice carrying specific viral resistance genes (FV-1, Rmcf) of linkage marker genes.